Granite, southwest Alaska and Aleutian Islands

Unit symbol: Tgw
Age range Tertiary, Oligocene, or older (38 to 23.03 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks in southern Alaska
Consists of several distinct plutons. The Windy Fork pluton (Bundtzen and others, 1997a; Gamble and others, 2013) in the McGarth and Lime Hills quadrangles consists of light-gray or grayish-pink biotite granite or aplite and peralkaline granite. The Windy Fork pluton yields K/Ar ages of 30.9±0.9 Ma (biotite), 29.7±0.9 Ma (hornblende) (Reed and Lanphere, 1972) and 23.5±0.7 Ma (pyroxene) (Gilbert and others, 1988a) and U/Pb ages on zircon are 31.8±0.6 and 30.9±0.4 Ma (D.C. Bradley, written commun., 2014); granite in the Lake Clark quadrangle (Nelson and others, 1983) is undated. Zircon dates on the Big River pluton of the Lime Hills quadrangle (Gamble and others, 2013) indicate an age of about 31 Ma (Jones and others, 2014). These plutons are included in the Tertiary phase of the Alaska-Aleutian Range batholith of Reed and Lanphere (1969, 1972). The second part of this map unit consists of albite granite found on Attu Island (Gates and others, 1971). The granite is gray, pale-pink, or purplish, has widely variable grain size, and locally contains euhedral gray-green feldspar (albite) phenocrysts up to 5 mm long. Quartz makes up about 20 percent of rock and, locally, granite may have as much as 5 percent green amphibole (Gates and others, 1971). The albite granite is surrounded by a halo of albitized and silicified basement rocks (unit TMzzu, here) that is as much as 2 miles wide. Age control is limited; albite granite blocks occur within keratophyric tuff in the basement rocks although the main intrusions cut through the basement rocks. Diabase dikes cut the granite, and boulders of the granite occur in the conglomeratic Chirikof Formation (unit Tms) on Attu Island, suggesting that the granite may be largely pre-Miocene. The granite on Attu Island was placed in the Attu Basement Series by Yogodzinski and others (1993)

Source map information

Source map Gamble, B.M., and Reed, B.L., 1996, Unpublished data
Symbol Twf
Unit name Windy Fork pluton
Description Peralkaline granite in two distinct phases, an alkali feldspar granite and granite. Overlaps into McGrath quadrangle. Anomalous U and Th near northern margin of pluton
Lithology Igneous

Correlated geologic units

Label Tpag
Description Peralkaline granite and related rocks
Geologic age Oligocene to Rupelian
Geologic setting Intrusive, granite
Lithology Form Importance
Alkali-feldspar-granite < Granitic < Plutonic < Igneous Major